Thomas Killian’s research group studies ultracold neutral plasmas and quantum degenerate atomic atomic gases. Both experiments start with laser-cooled and trapped neutral strontium. Laser-cooling is a powerful technique for producing and trapping atoms at temperatures as low as one millionth of a degree above absolute zero. Under these exotic conditions, matter behaves in fundamentally different ways, and the exploration of this regime teaches us about the basic laws of nature and lays the foundation for powerful new technological advances.

Our current focus is on creating and understanding strongly interacting, many-body systems. This is a grand challenge for many areas of physics, such as in quark-gluon plasmas in high energy and nuclear physics, and a rich spectrum of phenomena in condensed matter physics including high temperature superconductivity, exotic magnetic behavior, and superfluid helium. In the quantum realm, the highly entangled wavefunctions in these systems lead to the emergence of new physics, such as superconductivity. Ultracold atoms can reach this regime when they are cooled to temperatures on the order of nanokelvin and trapped in periodic potentials called optical lattices. This is the direction we are pursuing with our quantum degenerate strontium experiments. Ultracold neutral plasmas are strongly interacting because their thermal energy is less than the Coulomb energy between neighboring particles. This reverses the normal energy hierarchy found in plasmas and makes theoretical description much more difficult.

We are also developing new research directions creating strongly interacting states of strontium Rydberg atoms and manipulating biological structures with electromagnetic fields.

Ultracold Neutral Plasmas

Over 99% of the visible matter in the universe exists as plasma, in which neutral atoms have been ionized to produce free electrons and ions. Traditionally, neutral plasmas are relatively hot, such as the solar corona (1,000,000 K), a candle flame (1000 K), or the ionosphere around our planet (300 K). Using techniques of laser cooling, which originated in the atomic physics community, it is now possible to create ultracold neutral plasmas at temperatures as low as about 1 K. In a table-top apparatus, laser light traps and cools about 1 billion neutral atoms to a thousandth of a degree above absolute zero. A second laser illuminates the cloud with photons with barely enough energy to ionize the atoms and create the plasma.

Little is known about plasmas in this new regime, and they are difficult to describe theoretically because they are strongly interacting, which means that interactions cannot be treated as a small perturbation. We are currently measuring the collision rate in these systems, which is an unsolved problem that is important for modeling dense plasmas in thermonuclear devices and the cores of gas giant planets. We also recently developed a new technique for sculpting the density distribution of the plasma, which allows us to excite ion acoustic waves and create streaming plasmas and shock waves. This represents a new direction in the study of ultracold neutral plasmas that will allow us to probe basic plasma physics phenomena with unprecedented precision.

Quantum Degenerate Atomic Strontium

Our work on ultracold neutral atomic strontium focuses on the study of quantum degenerate gases. When atoms are cooled down to temperatures on the order of a millionth of a degree above absolute zero, they enter a regime dominated by quantum mechanics in which we can search for new phenomena and explore novel states of matter, or study the underlying physics of magnetism and superconductivity. Ultracold atoms serve as ideal model systems because the individual particles are well characterized, and their interactions can be controlled in ways that are not possible with traditional materials. Strontium in particular, has two valence electrons, which is unlike most other elements that have been coaxed into the quantum degenerate regime, and this has been predicted to lead to new types of collective properties at ultracold temperatures. In 2009 and 2010, we created the first strontium Bose-Einstein condensates and quantum degenerate Fermi gases, and we are currently developing new ways to control the interactions between atoms using laser light and putting atoms into optical lattices to access the strongly interacting regime.

A new project, in collaboration with Prof. Barry Dunning, is using ultracold strontium atoms in highly excited Rydberg states to study many-body systems with long-range interactions. Such systems are predicted to display exotic phenomena such as supersolidity and may enable the creation of three-dimensional solitons.

Manipulating Biological Systems with Electric and Magnetic Fields

In collaboration with Robert Raphael in Bioengineering and Biotech startup n3D Biosciences, Inc. (n3dbio.com), we are developing new techniques to manipulate cells and probe cell membranes with electromagnetic fields. A recent discovery is being commercialized by n3D as a new technology for levitating cells off the flat, two-dimensional surface of a petri dish and allowing them to grow in three dimensions. Three-dimensional structure and interactions with surrounding tissue, as experienced by cells in the body, are critical for normal cell growth and function. So this new technology has great potential for basic bio-medical research, drug discovery, and tissue engineering.

To learn more, contact Professor Killian at killian@rice.edu or stop by the lab in Brockman Hall B16.